1. Field of Invention
This invention relates to radiography. In a primary application the invention relates to the detection of images of x-ray flux with different energy spectra.
2. Description of Prior Art
Conventional radiography not only projects three dimensional body structures onto a two dimensional image but also integrates the transmission over a broad spectrum of x-ray photon energies. The projection over space and integration over the energy spectrum result in a superposition of all tissues and materials in the image making it difficult to visualize many structures of interest. For example, tumors and other soft tissue materials are often obscured by overlying bone. Also, large amounts of iodine containing contrast agents must be administered into blood vessels to make them visible in the presence of other body structures.
A solution to this problem lies in the physical fact that different materials have attenuation coefficients which have unique functions of energy. By making measurements at different regions of the energy spectrum, and combining them in an appropriate image processing system, specific materials can be removed or isolated. A preferred approach to the use of the energy spectrum information is described in U.S. Pat. No. 4,029,963 (1977), "X-ray Spectral Decomposition Imaging System," issued to R. E. Alvarez and A. Macovski. Here measurements made at two different regions of the x-ray energy spectrum are processed to calculate the photoelectric and Compton scattering components of the attenuation. These components, representing essentially atomic number and density, can be combined to represent different materials such as bone or soft tissue.
The performance of systems utilizing energy spectrum information depends critically on the energy selective measurements. In medical imaging systems, the important factor to consider is the noise for a given patient dose. Lower noise means that more subtle differences and structures can be discerned in the images. Patient dose should always be minimized in medical examinations. Noise and dose in processed energy spectrum images were studied by R. E. Alvarez and A. Macovski in their paper "Energy selective reconstructions in x-ray computerized tomography", Physics in Medicine and Biology, 1976, Vol. 21, pp. 733-744. They showed that the noise decreases with increased dose and with increased difference in average energies of the measurements. A larger difference in average energies produces lower noise for the same dose.
Approaches to energy selective measurement can be divided into two broad classes. In one class, the energy spectrum of the source is varied and the flux transmitted through the subject is measured with conventional, non-energy selective detectors. In the other class, the detectors themselves have energy selective capability to separate the transmitted photons into different effective spectra. The source spectrum can be changed, for example, by switching the voltage of an x-ray tube or by passing the x-ray beam through different x-ray attenuators. An example of an energy selective detector is a counting detector with pulse height analysis. Counting detectors are not practical in x-ray transmission medical imaging systems because the counting rates are too high. Other approaches need to be used with these systems.
An important consideration in choosing x-ray detectors is that, by far, the largest number of medical examinations are done with projection radiography utilizing area detector systems such as film. Other possible detectors are image intensifier tubes and scanned line detectors. Image intensifier tubes are used in medical radiography for special procedures requiring real time imaging of motion. Examples would be the motion of contrast agents though the circulatory system or the intestine. The image intensifier tubes have large vacuum envelopes. They are expensive and require complex mechanical support systems. Therefore they are not used in general radiography but only where real time imaging is indispensable. Scanned line detectors use linear electronic detector arrays to acquire an image by scanning it one line at a time. Line detectors have not been commercially accepted for several reasons. Freezing of motion requires that images be acquired in a short time resulting in high scan rates. These high rates produce stringent requirements on the electronics and the mechanical system. They also require much higher x-ray tube output than area detectors since they use only a small fraction of the tube's flux. Finally, scanned line detectors are not mechanically compatible with conventional x-ray equipment used for general radiography. Neither image intensifier tubes nor scanned line detectors are good choices for energy selective imaging in general radiography.
During medical examinations, film is placed in a thin protective box called a cassette. Nearly all medical equipment is designed to be used with these cassettes. If a detector can not fit into cassette holders, it would require special purpose systems that would be inconvenient for medical institutions. This would limit the market so compatibility with film cassette holders is a highly desirable feature of detectors for energy selective measurements.
Changing the source spectrum can provide energy selective measurements with conventional non-energy selective area detectors. To reduce image blur due to motion of the patient and body structures, both measurements must be made within a small fraction of a second (say about 0.1 second). Modern x-ray power supplies are controlled electronically by microprocessors so voltage switching in this period of time is relatively easy. It is also easy to change a small attenuator at the tube this rapidly. But making measurements within this time period with conventional area detectors has been difficult. Each spectral measurement must be recorded on a separate sheet. The only way to make separate measurements has been to physically move each sheet in and out of the path of the x-rays. These sheets are large, 35 cm by 43 cm for typical examinations, so mechanically moving them in this short a time is very difficult. Also, a mechanical changer does not fit into cassette holders of existing medical x-ray equipment.
U.S. Pat. No. 4,029,963 cited above describes an energy selective area detector, the passive layered detector cassette. The cassette consists of two conventional area detectors arranged in two layers so that x-rays forming the images pass first through a front detector then to a second back detector. The energy selectivity is provided by the x-ray filtering of the materials in the detectors. Since attenuation decreases with x-ray energy (except at a few discrete absorption edges) the mean distance before absorption is smaller for low energy than for high energy photons. Thus, the lower energies are preferentially absorbed in the front detector while the higher energies are preferentially absorbed in the back detector. If the material has an absorption edge within the energy region of interest, the opposite can be true. In this case the attenuation increases with energy so the photons detected by the front screen have a higher average energy than the those detected by the back screen.
In either case, the passive layered detector cassette has several important advantages. First, it works completely passively without any motion of the area detectors. A single exposure creates the energy selective images in both detectors simultaneously so there is no problem with motion. Another advantage is that the cassette has approximately the same size as an ordinary cassette. Thus it can be used in conventional x-ray equipment. Also, the approach can be used with any area detector.
The passive layered approach, however, has severe problems with selectivity compared to x-ray source spectrum switching. It must rely on the physical properties of x-ray attenuation to separate x-ray photons into different spectra. The paper by Alvarez and Macocski cited above shows the attenuation coefficient functions depend only on two constants and and, for some materials, on absorption edge energies. There are a very limited variety of attenuation functions and they are not especially suitable for selectivity. The problem is particularly difficult with detectors where only a small number of detector materials are in widespread commercial use. In order to enhance selectivity, materials with different atomic numbers should be used in the front and back layers. With different atomic number detector materials, layered detectors have poor selectivity. With only one material for both front and back screens, their selectivity is substantially worse.
U.S. Pat. No. 4,511,799 (1985) issued to P. J. Bjorkholm describes a passive layered cassette in which the x-ray beam is not perpendicular to the surface of the detectors. The only difference with the previous approach is that the path length is now the slant distance through the detector. The same attenuation can be obtained in a perpendicular incidence approach simply by using a thicker detector. Thus the non-perpendicular incidence angle does not increase the selectivity of the detector. The patent also describes the use of gas detectors. Gases have the same physical attenuation functions as solids only lower density. A solid detector can have the same x-ray attenuation as a gas if it is thin enough. A gas detector, therefore does not have inherently different energy selectivity than a solid detector.
U. K. Pat. No. 1,154,973 (1969) issued to W. K. French and E. W. Bauer, U.S. Pat. No. 4,578,803 (1986) issued to A. Macovski, and U.S. Pat. No. 4,626,688 (1986) issued to G. T. Barnes describe passive layered cassettes with an absorbing layer between the front and back detector. This can increase selectivity somewhat by absorbing low energy photons, that pass through the front detector, before they reach the back detector. Unfortunately, the absorbing layer also attenuates high energy photons so it can increase the required patient dose. The attenuation coefficient funtion of the absorber is also limited to the same set of physically realizable functions as the detectors. Its capability to increase selectivity is therefore limited.
U.S. Pat. No. 4,855,598 (1989) issued to M. Ohgoda and N. Nakajima describes a passive layered cassette with multiple detector layers. The paper by Alvarez and Macovski cited above shows that two spectrum measurements are sufficient to extract complete energy dependent information. Adding measurements does not add any new information. If the extra measurements are summed before processing then the result is the same as using thicker detectors in the approach of U.S. Pat. No. 4,029,963 since x-ray detectors are linear. If the data from the extra detectors are not used, the result is the same as using an absorbing layer.